Laser beam machining method

ABSTRACT

A laser beam machining method of processing a machining path having an acute angle in a CNC laser machining apparatus. A numerical control device (CNC) previously reads a machining program, to thereby detect an acute angle not greater than an allowable angle in the machining path. A machining operation is executed under normal machining conditions before a vertex (33) of the acute angle is reached and is stopped at the vertex (33) of the acute angle, and the region near the vertex (33) is cooled for a predetermined time by a cooling medium, whereby a lowering of the cut surface roughness due to an overheating by a laser beam (7) is prevented. After the cooling, the cutting work is executed under drilling conditions to continue the machining.

DESCRIPTION

1. Technical Field

The present invention relates to a laser beam machining method ofprocessing a machining path having an acute angle by a laser beam, andmore particularly, to a laser beam machining method of efficientlyeffecting an automatic cutting work even at an acute-angle processingportion which is susceptible to an influence of heat.

2. Background Art

In cutting work using a laser beam, the laser beam is applied to aportion of a material to be cut, and at the same time, an assist gas isblown onto that portion from a nozzle tip. When cutting a metallicmaterial, oxygen is generally used as the assist gas so that anoxidative reaction is used in conjunction with the heat-energyprocessing by the laser beam, and local heat processing work iscollected. When processing an acute-angle portion, however, thetraveling speed of a machining table is substantially lowered, and thusthe heat input in the stock per unit time is increased. Immediatelyafter the arrival of the laser beam at the turn, in particular, theacute-angle vertex portion is melted by local heat previously producedin the stock due to heat conduction before the cutting work, and for thesame reason, a considerable reduction in cut surface roughness occurs inthe region near the vertex portion.

The method used to cope with this problem varies in accordance withwhether the resulting product is on the side of an acute or narrowerangle or on the side of a greater angle.

If the product is on the acute-angle side (inside the cutting path), aloop processing method is used. In this method, the cutting work iscontinued until the turn of the acute angle is cleared, the machining iscontinued, forming a circular arc of a suitable size or a straight lineto return to the turn, and an advance for the originally intendedmachining is restarted. This method enables a substantial reduction ofthe machining speed at the acute angle, which is attributable to thecorrelation between the force of inertia of the machining table and aservomotor, to be lessened. If the size of the loop is increased, thenthe vertex of the acute angle will be cooled.

If the product is on the obtuse-angle side (outside the cutting path),on the other hand, a method is used such that the machining conditionsare changed before the turn is reached, to thereby attempt to minimizethe heat input, and the same operation is performed while the machiningis advanced for a suitable distance from the turn.

If the product is on the acute-angle side, however, it is necessary tocreate a program for the path of machining at the loop processingportion, in addition to the path of the originally intended machining,and thus the size of the loop must be changed in accordance with thethickness of the material to be machined and the size of the acuteangle. Moreover, since an unwanted machining is added, the material willbe unduly consumed.

Furthermore, if the product is on the obtuse-angle side it is verydifficult to set the machining conditions. Namely, know-how based onexperience and perception is required to determine the position at whichthe machining conditions should be changed or the position at which themachining conditions should be restored to the original conditions,during the progress of the machining operation. Moreover, since the heatinput is restricted during the machining, the operation is limited to anextent such that the cutting work becomes impossible. Also, since manychanges are made in the machining conditions of this method, acomplicated machining program is required, and thus a problem arises inthe use thereof.

DISCLOSURE OF THE INVENTION

The present invention has been contrived in consideration of thesecircumstances, and an object thereof is to provide a laser beammachining method of automatically effecting an acute-angle machiningwithout the need for a complicated machining program or the machining ofunnecessary portions.

To solve the above problems, according to the present invention, thereis provided a laser beam machining method of processing a machining pathhaving an acute angle in a CNC laser machining apparatus, which laserbeam machining method comprises previously reading a machining programto thereby detect that the acute angle of the machining path is notgreater than a preset allowable angle, moving a workpiece for cuttingwork to the vertex of the acute angle under normal machining conditions,automatically stopping the application of a laser beam when the vertexis reached, ejecting a cooling medium for a predetermined time, drillingthe workpiece under set drilling conditions after the passage of thepredetermined time, and continuing the cutting work under the normalcutting conditions.

A numerical control device (CNC) previously reads the machining program,to thereby detect that the acute angle is not greater than the allowableangle in the machining path. A machining operation is executed under thenormal machining conditions before the vertex of the acute angle isreached and is stopped at the vertex of the acute angle, and the regionnear the vertex is cooled for a predetermined time by the coolingmedium, whereby a lowering of the cut surface roughness due to anoverheating by the laser beam is prevented. After the cooling thecutting work is executed under drilling conditions, to continue themachining operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a laser beam machining method accordingto the present invention; and

FIG. 2 is a block diagram showing an arrangement of a CNC laserapparatus for effecting the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

One embodiment of the present invention will now be described withreference to the drawings.

FIG. 2 is a block diagram showing an arrangement of a CNC laserapparatus for effecting the present invention. In FIG. 2, a processor 1reads out a machining program from a memory 10 in accordance with acontrol program stored in a ROM (not shown), and controls the operationof the whole CNC laser apparatus. An output control circuit 2, whichcontains a D/A converter, converts an output command value deliveredfrom the processor 1 into a current command value and delivers thecurrent command value as an output. An exciting power source 3 producesa high-frequency voltage by a switching operation after rectifying thecommercial power source, and supplies a discharge tube 4 with ahigh-frequency current corresponding to the current command value.

A laser gas 19, which circulates in the discharge tube 4, is excited byan electric discharge produced when the high-frequency voltage from theexciting power source 3 is applied. A rear mirror 5 is a germanium (Ge)mirror with a reflection factor of 99.5%, and an output mirror 6 is azinc selenide (ZnSe) mirror with a reflection factor of 65%. Thesemirrors, which constitute a Fabry-Perot resonator, amplify a 10.6-μmlight beam emitted from excited laser gas molecules, and a part of thelight beam is output as a laser beam 7 from the output mirror 6 to theoutside.

The course of the output laser beam 7 is changed by a bender mirror 8,to be converged onto a spot of 0.2 mm or less by a light converging lens9 and applied to the surface of a workpiece 31.

The memory 10, which is a nonvolatile memory for storing the machiningprogram and various parameters, etc., is formed of a CMOS backed up by abattery. In addition, a ROM is used for storing a system program and aRAM is used for temporarily storing data; these memories are omitted inFIG. 2.

In response to a command from the processor 1, a position controlcircuit 11 controls the rotation of a servomotor 13 by using a servoamplifier 12, and controls the movement of a table 16 through a ballscrew 14 and a nut 15, to thereby control the position of the workpiece31. Although only one axis is shown in FIG. 2, a plurality of controlaxes are actually used. A CRT or liquid crystal display device is usedas a display unit 18.

A Roots blower is used as a blower 20 which circulates the laser gas 19through cooling units 21a and 21b. The cooling unit 21a is used to coolthe laser gas 19 heated to high temperature by laser oscillation, andthe cooling unit 21b is used to remove compression heat produced by theblower 20.

A cooling medium control circuit 22 controls a cooling medium 26 byturning on and off a solenoid valve 23. Gas, water, or steam, etc. maybe used as the cooling medium 26.

A power sensor 24, which is formed of a thermoelectric or photoelectricconversion element, receives the laser beam 7 partially transmitted anddelivered through the rear mirror 5, and measures the output power ofthe laser beam 7. An A/D converter 25 converts the output of the powersensor 24 to a digital value, and inputs the digital value to theprocessor 1.

FIG. 1 illustrates a laser beam machining method according to thepresent invention. When cutting the workpiece 31 from a cutting startpoint 32 through an acute-angle vertex 33 to a cutting end point 34, thecutting start point 32 is drilled under drilling conditions, and theworkpiece 31 is moved so that the acute-angle vertex 33 is reached afterthe drilling work is completed.

Before the acute-angle vertex 33 is reached, a numerical control device(CNC) previously reads the machining program and executes an automaticangle discrimination. If the angle is narrower than a preset allowableangle, a signal for stopping the application of the laser beam is outputto stop the laser beam application for a predetermined time when theacute-angle vertex 33 is reached, and at the same time, the region nearthe acute-angle vertex 33 is cooled by the cooling medium 26, such asgas, steam, or water. After the passage of the predetermined time, theacute-angle vertex 33 is drilled under the same drilling conditions asfor the drilling at the cutting start point 32. After the end of thisdrilling work, the workpiece 31 is moved so that the cutting end point34 is reached under the same cutting conditions as in the case where theworkpiece 31 is moved from the cutting start point 32 to the acute-anglevertex 33. The application of the laser beam is stopped when the cuttingend point 34 is reached.

When a 16-mm thick workpiece was subjected to an acute-angle machiningof 30° by using a CO₂ gas laser having a 2-kW output, according to thepresent invention, the same surface roughness as in the case of portionsother than the acute-angle portion was obtained with the use of air asthe cooling medium at a cooling time of about 2 seconds, withoutchanging the other cutting conditions.

According to the present invention, as described above, the machiningpath having an angle not greater than the allowable angle is detected bythe numerical control device, the application of the laser beam isstopped at the acute-angle vertex, and the machining operation isrestarted after the cooling by the cooling medium. Therefore, theoperator can create a cutting program while taking into account only themachining path.

As a result, the operator can perform the acute-angle cutting work,which conventionally has often resulted in a failure, while ignoring theacute angle, and there is no need for loop processing, and thus theusage of stock can be minimized.

I claim:
 1. In a laser beam machining method of processing a machiningpath having an acute angle in a CNC laser machining apparatus, the laserbeam machining method comprising:previously reading a machining program,to thereby detect that the acute angle of the machining path is notgreater than a preset allowable angle; moving a workpiece for cutting tothe vertex of the acute angle under normal machining conditions;automatically stopping the application of a laser beam when said vertexis reached; ejecting a cooling medium for a predetermined time; drillingthe workpiece under set drilling conditions after the passage of saidpredetermined time; and continuing the cutting work under the normalcutting conditions.
 2. A laser beam machining method according to claim1, wherein said cooling medium is one of gas, water, and steam.
 3. Alaser beam machining method according to claim 1, wherein said allowableangle is stored as a parameter of the machining conditions.